STEM coupled with the optical system was used for the investigation of
the early oxidation on the surface of Cr. Cr thin films (30 – 1000Å) were
prepared by evaporation onto the polished or air-cleaved NaCl substrates at
room temperature and 45°C in a vacuum of 10−6 Torr with
an evaporation speed 0.3Å/sec.
Rather thick specimens (200 – 1000Å) with various preferred orientations
were used for the investigation of the oxidation at moderately high
temperature (600 − 1100°C). Selected area diffraction patterns in these
specimens are usually very much complicated by the existence of the
different kinds of oxides and their multiple twinning. The determination of
the epitaxial orientation relationship of the oxides formed on the Cr
surface was made possible by intensive use of the optical system and
microdiffraction techniques. Prior to the formation of the known
rhombohedral Cr2O3, a thin spinel
oxide, probably analogous to γ
-Al203 or γ
-Fe203, was formed. Fig. 1a
shows the distinct epitaxial growth of the spinel (001) as well as the
rhombohedral (125) on the well-oriented Cr(001) surface. In the case of the
Cr specimen with the (001) preferred orientation (Fig. 1b), the rings
explainable by spinel structure appeared as well as the well defined
epitaxial spots of the spinel (001). The microdif fraction from 20A areas
(Fig. 2a) clearly shows the same pattern as Fig. Ia with the weaker oxide
spots among the more intense Cr spots, indicating that the thickness of the
oxide is much less than that of Cr. The rhombohedral
Cr2O3 was nucleated preferably
at the Cr(011) sites provided by the polycrystalline nature of the present
specimens with the relation Cr2O3
(001)//Cr(011), and by further oxidation it grew into full coverage of the
rest of the Cr surface with the orientation determined by the initial
nucleation.
